Question

How is BMI calculated?

Hint:

While BMI is a useful population-level screening tool, it does not measure body fat percentage directly. It can misclassify highly muscular athletes as "overweight" because muscle tissue is significantly denser than fat.

Answer 1

Step 1: Defining Body Mass Index (BMI):
Body Mass Index (BMI), also known as the Quetelet Index, is a simple, internationally recognized heuristic proxy value used to classify an individual's weight status relative to their height. It categorizes individuals as underweight, normal weight, overweight, or obese.

Step 2: The Mathematical Formula:
BMI is calculated by dividing an individual's body weight in kilograms by the square of their height in meters: $$\text{BMI} = \frac{\text{Weight (kg)}}{\left[\text{Height (m)}\right]^2}$$

Step 3: Step-by-Step Calculation Example:
Let's calculate the BMI of an individual who weighs $70\text{ kg}$ and stands $1.75\text{ meters}$ ($175\text{ cm}$) tall:

• Square the height in meters: $$1.75 \times 1.75 = 3.0625\text{ m}^2$$
• Divide weight by squared height: $$\text{BMI} = \frac{70}{3.0625} \approx 22.86\text{ kg/m}^2$$

Step 4: Standard Clinical Classifications (WHO Standards):

• Underweight: $\text{BMI} < 18.5\text{ kg/m}^2$
• Normal Weight: $\text{BMI} = 18.5\text{ to }24.9\text{ kg/m}^2$ (or $18.5\text{ to }22.9\text{ kg/m}^2$ for Asian populations)
• Overweight: $\text{BMI} = 25.0\text{ to }29.9\text{ kg/m}^2$ (or $23.0\text{ to }24.9\text{ kg/m}^2$ for Asian populations)
• Obese: $\text{BMI} \ge 30.0\text{ kg/m}^2$ (or $\ge 25.0\text{ kg/m}^2$ for Asian populations)